1. Field of the Invention
The present invention relates to a signal modulation circuit and a signal modulation method and, especially, a signal modulation circuit and a signal modulation method for modifying a signal based on a phase signal and an amplitude signal extracted from a modulating signal and, for example the signal modulation circuit and the signal modulation method for modifying a signal in the transmission unit of a cellular phone.
2. Description of the Related Art
Some of conventional signal modulation circuits and signal modulation methods are described in (1) “An Up-Conversion Loop Transmitter IC for Digital Mobile Telephones” (Siemens Microelectronics/1998 IEEE ISSCC SP 23.1), and (2) “A 2.7V 900 MHz/1.9 GHz DUAL-BAND TRANSCEIVER IC FOR DIGITAL WIRELESS COMMUNICATION” (Rockwell International/1998 IEEE CICC).
The modulation method described in the reference documents (1) and (2) may not apply to the modulation whose amplitude signal varies, since the method adopts a circuit for signal processing of sole phase signal only by phase synchronization loop.
In order to cope with a modulation form where the amplitude signal changes, for instance, π/4 shift QPSK (π/4 shift Quadrature Phase Shift Keying), it is necessary to adopt a construction as shown in FIG. 2 of the reference document (3) “Transmitter architectures [GSM hand set]” (1998 IEEE CDDCH). However, in this construction, modulated wave contains wave B as shown in FIG. 1 as out of band undesired component, since frequency of a modulating signal is converted without processing by the phase synchronization loop. It is necessary to use many voluminous and expensive filters to remove such out of band undesired component. There is a problem of increasing disadvantageously volume and cost of signal modulation system.